What will happen when you place a fake speedsign on a highway?
https://www.technologyreview.com/s/530276/hidden-obstacles-for-googles-self-driving-cars/


  Google’s cars can detect and respond to stop signs that aren't on its map, a feature that was introduced to deal with temporary signs used at construction sites. But in a complex situation like at an unmapped four-way stop the car might fall back to slow, extra cautious driving to avoid making a mistake.


It's highly probable they would slow down with current technology, as they can detect temporary signs and are designed to use slow speed in complex cases.

If it was a true temporary sign (e.g. road repair), how can it make the distinction? It probably would be worse to ignore a slow down sign than slow down at a fake one.

IMHO, the problem there is with the joke in the first place, as some humans might slow down too.